This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Some audio/video receiver modules, which may be incorporated into display devices such as televisions, have been designed with an audio output digital to analog (D/A) clock that is locked to a video output D/A clock. This means that the audio clock and video clock cannot be controlled separately. A single control system may variably change the rate of both clocks by an equal percentage. In some of these systems, a clock recovery system may match the video (D/A) clock to the video source analog to digital (A/D) clock. The audio output D/A clock may then be assumed to match to the audio source A/D clock. This assumption is based upon the fact that broadcasters are supposed to similarly lock their audio and video clocks when the source audio and video is generated.
Although the Advanced Television Systems Committee (ATSC) specification requires broadcasters to lock their video source A/D clock to their audio source A/D clock, there have been instances where these clocks were not locked. Failure of broadcasters to lock the clock of transmitted audio source material with the clock of transmitted video source material may result in a time delay between when the audio presentation should be occurring and when the audio is actually presented. This error, which may be referred to as lip synchronization or lip synch error, may cause the sound presented by the audio/video display device to not match the picture as it is displayed. This effect is annoying to many viewers.
When the audio/video clock recovery is driven by matching the video output rate to the video input rate, the only way to compensate for lip synch error is to time-manipulate the audio output. Because audio is a continuous time presentation, it is difficult to time-manipulate the audio output without have some type of audible distortion, mute, or skip. The frequency of these unwanted audible disturbances is dependent upon the frequency difference between the relative unlocked audio and video clocks at the broadcast station. ATSC sources have been observed to mute the audio every 2-3 minutes. The periodic muting of the audio signal may produce undesirable results to the viewer of the television.
One method of maintaining lip synch between digitized audio and video may include making specific measurements of the audio and video buffers and interpreting this information to determine the lip synch offset in time. In a streaming application, however, it is difficult to know the exact buffer level of a buffer because of the nature of data streams (e.g., video and audio data streams) and receiving buffers. The buffer level cannot be easily read because it is always changing. In other words, merely reading the buffer level at a given point in time may not prove accurate because the buffer level may be constantly changing over time.
One approach to determining the operating point of a buffer is to average buffer levels over time. However, averaging the buffer level may lead to skewed results if the window of time that is observed includes partial buffer fills or drains, or if playback is interrupted, for example, by an early fill midway.